1. Field of the Invention
The present invention relates to an optical multiplexer equipped with a semiconductor laser device and applied to optical communication fields. More particularly, the present invention relates to an optical wavelength division multiplexing module equipped with a surface emitting semiconductor laser device and a communication system using the same.
2. Description of the Related Art
The wavelength division multiplexing (WDM) communication system has been developed in which light signals having different wavelengths are combined or multiplexed and are then transmitted over a single optical fiber, so that an increased optical transmission capacity can be obtained. A vertical-cavity surface-emitting laser (hereinafter, simply referred to as VCSEL) is suitable for the optical source of the optical wavelength division multiplexing.
A conventional optical WDM communication module is equipped with multiple VCSEL chips of different wavelengths housed and arrayed in a metal case. Laser lights of different wavelengths emitted from the VCSEL chips are combined in a single optical fiber via optical components such as a collimating lens and a condenser lens. A module on the receive side separates the multiplexed lights from the optical fiber into individual lights on the wavelength basis, and senses the separate lights of the respective wavelengths by means of light-receiving elements or the like.
Japanese Patent Application Publication No.11-340565 (Document 1) discloses, in FIG. 1, an optical coupling element and an optical coupling device. Lights emitted from an emitting part 11 of a surface-emitting type single-color VCSEL array 10 are collimated by a lens array 20 including multiple lens parts 21. The lights thus collimated are coupled in an optical fiber 40.
Japanese Patent Application Publication No. 6-97578 (Document 2) discloses a multi-wavelength semiconductor laser array for WDM in which the oscillation frequency is varied by changing the cross section of the laser waveguide integrated on a single substrate so that the array can oscillate with the multiple wavelengths.
However, there is a problem to be solved in the optical WDM communications using the VCSEL as the optical source. The device disclosed in Document 1 needs the collimating lenses 10 (lens array 20) and the condenser lens 30 to couple the lights from the source array 10 in the optical fiber, and thus needs optical alignment of two kinds of lenses as shown in FIG. 1 of Document 1. It is therefore necessary to precisely adjust the components. This increases the cost.
The technique disclosed in Document 2 changes the cross section of the waveguide or the resonator to thus vary the oscillation frequency. However, the wavelength can be changed within only a narrow range, and there is difficulty in widening the variable wavelength range. This allows only limited applications to the optical source in WDM communications. The optical WDM can be categorized into DWDM (Dense Wavelength Division Multiplexing) and CWDM (Coarse Wavelength Division Multiplexing). The DWDM is used for relatively expensive communications that employ wavelengths at short intervals and use many laser devices. The CWDM is used in relatively less-expensive, short-distance communications that employ wavelengths at long intervals and use four to eight laser devices. The CWDM requires the intervals equal to at least 20 nm. However, it is difficult to realize the wave intervals as long as 20 nm by the optical source described in Document 2, which is not suitable for CDMW.